1. Field of the Invention
The present invention relates, generally, to semiconductor devices and, more particularly, to InP-based devices. Specifically, the present invention relates to improved substrate structures for InP-based devices which are mechanically rugged, inexpensive lightweight.
2. Description of the Prior Art
Semiconductor devices have wide versatility and applications. Use of semiconductors for photovoltaic, integrated circuit, and optoelectronic applications has grown immensely in recent years, and adaptations of such devices to space environments have received considerable attention. One common problem with conventional semiconductors is their lack of radiation resistance required to insure degradation-free operation in space. This problem is especially troublesome when considering space photovoltaics where conventional Si solar cells degrade with time. Thus, alternate semiconductor materials have been investigated to overcome these and other nonsolar application problems.
Indium phosphide (InP) is an attractive III-V semiconductor for a variety of electronic device applications because of the large number of lattice-matched III-V ternary and quaternary materials available, for example GaAsSb, GaInAs, AlAsSb, and AlInAs. In addition to being lattice matched, these compounds offer a wide range of band gaps, which aid in the design of complex device structures. InP is also considered a prime candidate for space photovoltaic applications because of its superior radiation hardness and demonstrated high efficiencies. Additional applications include monolithic integration of InP based optoelectronic devices with Si- or GaAs-based devices. However, the primary impediments to such applications have been high cost, low availability, and inherent fragility of the InP substrates. Consequently, research efforts have been under way to investigate possible InP arrangements that can overcome some of the above deficiencies.
U.S. Pat. No. 4,591,654 discloses an InP solar cell having a Si substrate with a GaInP layer disposed between the Si and the InP layers. The GaInP layer is a graded layer having a higher concentration of Ga on the Si side and a higher concentration of In on the InP side. Unfortunately, this arrangement is limited to the use of Si, does not take into account the potential problems associated with thermal expansion coefficient differences between Si and InP, and does not deal effectively with the dislocation propagation problem from the Si substrate into the InP layer.
Various technical articles disclose other attempts to solve the problem of growing InP on a foreign substrate. InP layers have been deposited directly onto Si and GaAs. However, the InP layers thus formed contain high dislocation densities resulting from the large lattice mismatch. These dislocations severely degrade device performance. For example, although some InP/Si solar cells have been formed using the above techniques, they generally have had poor performance characteristics. Thus, there is still a need for a structure which allows device quality InP to be grown on alternate substrates in a manner to effectively reduce the dislocation generation propagation problem stemming from lattice mismatch.